About this module
Many digesters in Ghana stopped working because nobody sized them properly, checked the build, trained the owner or came back to service them. Level 2 trains the person who does those things.
Who it is for. People who have finished Bio Digester Technology (Level 1), or who can show the same skills. National Proficiency II covers varied work done under limited supervision.
Standards status. Ghana had a competency-based biogas standard for artisans at NP II. It covered biogas principles, health and safety for each trade, dome construction, plumbing, steel bending, formwork and the biogas business, but it expired in December 2021. This module is not a CTVET curriculum.
Bridge from Level 1: what you already know
- Never enter a digester, pit or manhole, even to rescue; test for leaks with soapy water, never a flame
- Volume = daily feed × retention days (40 days for dung, 60 for toilet waste)
- Gas yields for cattle, pig and poultry dung; about 0.3 m³ per person per day for cooking
- Mix feed 1:1 with water; balance carbon and nitrogen; what never goes in
- Fixed dome, floating drum and bag digesters; the fixed-dome build order
- Pipe slope with a water trap at the low point
- The common faults table; a sour digester and pH
- Dilute slurry 1:1; toilet slurry never on raw-eaten crops
Level 1 did not cover large institutional digesters, electricity from biogas, or building without a qualified builder. Level 2 covers the first two and teaches you to supervise a qualified builder.
How the module works
Each unit has learning outcomes, performance criteria, the range and the evidence your assessor needs. You are judged Competent or Not Yet Competent. About 70% of the time is practical, following CTVET's Oil Palm programme split of about 30% school and 70% workplace.
Safety first
The never-enter rule. You never enter a digester, pit, manhole or tank, even to rescue someone. All servicing is done from outside with hooks, poles and valves. Any work inside is done only by qualified people with breathing apparatus and confined-space procedures.
- Methane explodes when it makes up 5–15% of the air. Most accidents happen during start-up, venting, shutdown and maintenance, so write a step plan for each.
- Hydrogen sulphide (H2S) smells of rotten eggs at tiny amounts, but at higher levels it destroys your sense of smell and can kill within minutes. Carry a personal gas detector on farm and institutional sites.
- No smoking, no flames, and spark-free tools near the gas space.
- Rescue drill: shout for help, keep everyone out, ventilate from outside, call emergency services. Practise every term.
Unit 1 · Design and size a system for a client
A digester that is too big starves; one that is too small cannot meet the cooking need. Start with the client's waste and the client's cooking.
Competency standard
| Learning outcome | Performance criteria — the trainee… |
|---|---|
| 1.1 Survey feed and demand | records available waste and the client's cooking hours |
| 1.2 Size the system | calculates digester volume, gas storage and daily gas |
| 1.3 Choose a type | chooses and justifies a digester type |
Range. Two real clients: a farm and an institution. Evidence. Survey forms; sizing sheets.
Sizing rules
- Digester volume = daily feed (with water) × retention time.
- In fixed domes, gas storage is about 25–30% of the digester volume. Do not build a single digester larger than about 100 m³; two or more digesters in series are easier to repair and handle changing loads.
- Farm plants usually have a digester-to-gas-holder ratio between 3:1 and 10:1, most often about 5:1 to 6:1.
- Biogas is 50–75% methane, with about 6 kWh of energy per m³. One litre of kerosene equals about 2 m³ of biogas, and 1 kg of charcoal about 0.5 m³.
Institutions
- A guide for prisons sizes about 100 litres of digester per person. 200 people need about 20 m³.
- Each person produces about 3.3–4.9 litres a day of faeces, urine and flush water, and prisons measured 26–62 litres of gas per person per day, more where kitchen waste was added.
Toilet retention time. Level 1 used 60 days for toilet waste; the prison guide sizes at 30 days. Longer retention kills more germs. Use the longer figure and always add post-treatment (Unit 6).
Size it
A school has 300 boarders. Using 100 litres per person, what digester volume is needed? If gas storage is 25% of that, how big is the gas store?
Pineapple waste worked well mixed with cow dung in West African trials, a lead for fruit processors.
Two designs
- Survey a farm and an institution: waste per day, cooking hours, space.
- Size each system and choose a type.
- Have your trainer check and sign the designs.
Fuel bill
Ask a client what they spend on LPG, charcoal or firewood each month.
Unit 2 · Site, cost and supervise construction
Most failures are built in on day one: wrong slope, no traps, rainwater flowing in. A good supervisor catches them.
Competency standard
| Learning outcome | Performance criteria — the trainee… |
|---|---|
| 2.1 Assess the site | checks soil, water table, flooding and slopes |
| 2.2 Cost the job | prepares a bill of quantities and a quote |
| 2.3 Check the build | uses a quality checklist during construction |
Range. One build or a detailed mock build. Evidence. Site report; quote; completed checklist.
Why Ghana's digesters failed
Of 50 plants surveyed in Ghana, only 44% worked well; 28% did not work at all. Causes included no dung, broken gas balloons, no maintenance service, poor operating knowledge, leaks, toilet odours and wrong pipe connections. Another study reported that of about 50 plants visited, only 25 ran without problems.
Common design faults
- Inlet slope less than 5% blocked the inlet.
- No fall from the overflow chamber let rainwater flow back.
- No water traps blocked the gas pipes.
Inspect the site in the rainy season for standing water. A digester can last 20 years, pipes about 7.
Types and costs
- Ghana uses fixed domes, Puxin digesters with a removable gas holder, and floating drums. CSIR-IIR has also designed baffled digesters built like septic tanks and portable precast units.
- A 2021 Ghana study priced a 10 m³ fixed dome with a filter bed at GH¢26,200–28,000. Minor repairs cost GH¢250–600 and major repairs GH¢1,200–2,000.
- Global costs range widely, from cheap bag digesters to built plants.
Old prices. The 2021 figures are out of date. Always get three local quotes for materials and labour.
Build checklist
- Pit shored and safe; PPE for each trade
- Inlet slope at least 5%
- Overflow drains away from the digester
- Dome gas-tight; gas neck and outlet sealed
- Water trap at every low point
- Backfill compacted
- Pressure test passed before use
Quote and check
- Write a site report for one of your Unit 1 designs.
- Prepare a bill of quantities and a quote.
- Use the checklist on a real or mock build.
Price list
Get current prices for cement, blocks, sand, GI pipe and a biogas stove.
Unit 3 · Commission and hand over
Starting a digester and training the owner are what make it last.
Competency standard
| Learning outcome | Performance criteria — the trainee… |
|---|---|
| 3.1 Start the digester | fills, seeds, vents first gas and pressure-tests |
| 3.2 Hand over | trains the user and hands over the warranty and log |
Range. One new or restarted digester. Evidence. Commissioning record; signed handover.
Start-up
Fill with a 1:1 dung and water mix. Burnable gas comes in 1–4 weeks; adding slurry from a working plant speeds this. The first gas is mostly carbon dioxide and will not burn, so vent it outdoors. Do not fill to the very top, or slurry will be pushed into the gas pipe.
Venting. Vent first gas outdoors, away from people and flames. Never test-burn at the digester.
After-sales
Nepal's biogas programme gave a 6-year structural guarantee, a 1-year guarantee on appliances, yearly service visits and user training about 6 months after start-up, with women trained first.
Commission
- Fill and seed a digester; record dates and amounts.
- Pressure-test the pipe and valves.
- Train the owner and sign a handover sheet.
Handover sheet
Write a one-page owner's guide in plain language, with pictures.
Unit 4 · Pipe, clean and use the gas
Good gas at the right pressure gives a clean blue flame. Dirty gas eats stoves; wet pipes block.
Competency standard
| Learning outcome | Performance criteria — the trainee… |
|---|---|
| 4.1 Size gas pipes | sizes pipe for length and flow; places traps and valves |
| 4.2 Clean and use gas | installs an H2S scrubber; tunes stoves |
| 4.3 Read pressure | measures and interprets gas pressure |
Range. One gas line. Evidence. Pipe plan; pressure readings.
Pipes
For small and medium plants, ½ or ¾ inch pipe is fine up to about 30 m. A ¾ inch pipe can serve a small plant up to about 1.5 m³ per hour. Nepal banned thin PVC gas lines because they burst; the main valve was the most common leak.
Pressure and appliances
- Burners work at about 75–150 mm water column, using 150–450 litres of gas an hour; lamps use about as much gas as a 60 W bulb and are very inefficient.
- Fixed-dome pressure rises and falls as gas is stored, so burners are harder to tune. Floating drums give steady pressure.
- To convert an LPG stove: widen the jet to about 3 mm, burner holes to 4 mm, and add an air flap.
Cleaning the gas
H2S corrodes stoves and metal. Use an iron-oxide scrubber, ideally two (one working, one regenerating). A steel-wool bed removed about 95% of H2S in a laboratory study.
Electricity
One m³ of biogas gives only about 1.5 kWh of electricity, and generators usually pay only above about 50 kW. Generators need H2S removal first and a qualified electrician.
Valuing the gas
The LPG price floor was GH¢10.85 per kg from 1 September 2026. Use this to show clients what their gas saves. Gas in bags sold at a Kenyan slaughterhouse for about half the price of the same amount of LPG.
Gas line
- Plan a gas line with pipe sizes, valves and traps.
- Fit a scrubber.
- Read pressure with a U-tube manometer morning and evening for a week.
Gas value
Estimate a client's monthly LPG saving using today's LPG price.
Unit 5 · Troubleshoot and service
A digester with a service plan keeps working. One without it often fails within a few years.
Competency standard
| Learning outcome | Performance criteria — the trainee… |
|---|---|
| 5.1 Diagnose low gas | checks feed, leaks, water and biology in order |
| 5.2 Repair from outside | repairs valves, pipes, scum and slurry in the line from outside |
| 5.3 Keep service records | records every visit |
Range. At least three digesters. Evidence. Diagnosis sheets; service records.
Service makes the difference
In East Africa, 27% of digesters built from 2009 to 2013 were not working by 2016. After a call centre and after-sales service were added, 77–94% were working. India's scheme gives a two-year installer warranty and three years of routine maintenance with several visits a year by trained local mechanics. One institutional plant failed because knowledge was lost when staff changed.
Low gas: check in this order
- Feed: enough, mixed right, every day?
- Leaks: soapy water on every joint, especially the main valve
- Water: traps drained? slurry in the line?
- Biology: sour smell, pH, harmful inputs?
- Pressure: manometer reading compared with last visit
Service round
- Visit three digesters with your trainer.
- Diagnose any problem using the checklist.
- Repair what can be done from outside and record each visit.
Service plan
Write a 12-month service plan for one client.
Unit 6 · Toilet-linked and institutional systems
Toilet digesters protect health only if the effluent is handled safely and the rules are kept.
Competency standard
| Learning outcome | Performance criteria — the trainee… |
|---|---|
| 6.1 Size for people | sizes systems by population |
| 6.2 Handle effluent safely | applies sanitation rules and plans post-treatment |
| 6.3 Set operating rules | writes rules for the institution |
Range. One school, prison, hospital or estate. Evidence. Sizing sheet; effluent plan; operating rules.
Ghana experience
- Ghana's early biogas plants were mostly at hospitals and slaughterhouses. The Kumasi Abattoir plant cost €1.28 million.
- The EPA funded biogas plants at senior high schools in Greater Accra.
- More than 50,000 biodigester toilets have been built under the Greater Accra sanitation project since 2016, and Kumasi trains students to build them.
Effluent is not safe yet
- Digestion did not remove worm eggs; the effluent was not safe even for restricted irrigation without post-treatment such as settling, drying beds or hot composting near 70 °C.
- A Ghana study of biodigester toilets found large drops in pollution, but E. coli fell only 63% and was found in soil at nearly all sites.
Effluent rules. Wear gloves and boots. Post-treat effluent. Never use it on crops eaten raw. Keep systems away from wells and flood-prone ground.
Operating rules
At most 3 litres per flush, no chlorine cleaners into toilets, and disinfectant run-off kept out of the bowl. Name the person responsible.
Institution plan
- Size a system for an institution.
- Write an effluent post-treatment plan.
- Write a one-page operating rules poster.
Visit
Visit a school, hospital or estate with a biodigester toilet. Ask who maintains it.
Unit 7 · Slurry, money and the biogas business
Slurry is fertiliser, gas is fuel saved, and service is a business.
Competency standard
| Learning outcome | Performance criteria — the trainee… |
|---|---|
| 7.1 Use slurry well | stores and applies slurry without losing nitrogen |
| 7.2 Calculate payback | calculates payback; explains carbon finance honestly |
| 7.3 Plan the business | plans a service business and markets it |
Range. One client. Evidence. Slurry plan; payback sheet; business plan.
Slurry
Digestion turns up to half the organic nitrogen into a form plants can use at once. More than half of it can be lost from uncovered slurry within a month; covering cuts the loss about tenfold. Work slurry into the soil straight after spreading. Slurry also fertilises fish ponds.
Money
- A 2021 Ghana study found a 10 m³ digester worthwhile, counting saved cesspit emptying, saved septic tank building, fuel savings and fertiliser.
- A study of Ghanaian farm households found digesters paid over 20 years where they suited the farm, with slurry use central.
Payback (years) = cost of the system ÷ yearly savings
Payback
A system costs GH¢30,000. It saves GH¢600 a month in LPG and GH¢1,500 a year in fertiliser. What is the payback?
Carbon finance
Ghana set up a carbon market office in 2022. A programme with a Swiss carbon deal plans about 20,000 home biogas systems for pig farmers in nine regions, including the Western Region. Carbon credit standards cover household digesters, but experts warn that credits are sometimes claimed for broken or unused digesters.
Be honest about carbon money. Do not promise clients carbon income. Programmes pay installers and subsidise systems; the farmer's share is often not public.
Location settings that give good returns
- Pig, poultry and cattle farms with daily manure
- Boarding schools, prisons, hospitals and estates
- Abattoirs and fruit processors
- Communities with costly septic emptying
- Areas served by carbon-funded biogas programmes
Business ideas and ingenuity
- Yearly service contracts with a warranty
- A district network of trained local mechanics
- Converting septic tanks to digesters
- Bagged gas for nearby food sellers
- Covered or composted slurry sold to growers
- Becoming a local installer or service agent for funded programmes
Market yourself
- Show working systems you service; ask happy clients to speak for you.
- A clear price list for visits and repairs.
- WhatsApp reminders before service dates.
Accessibility
Persons with disabilities in Ghanaian agriculture report a lack of land, money and tools, and negative attitudes, and some have been trained in agri-tech business. Design, quoting, records, follow-up calls, slurry sales and user training suit many people with disabilities. Use colour-coded valves, large-print logbooks, recorded instructions in Fante or Twi, and visual gas alarms for deaf workers.
Business plan
- Write a slurry use plan for a client.
- Calculate payback for a client.
- Write a one-page service business plan.
Programme contact
Find out whether a biogas programme operates in your district and how to become an agent.
Glossary
| Term | Meaning |
|---|---|
| Commissioning | Starting a new digester safely |
| H2S scrubber | A filter that removes hydrogen sulphide from gas |
| Manometer | A U-tube that measures gas pressure |
| Post-treatment | Extra treatment of effluent to kill germs |
| Retention time | How long feed stays in the digester |
| Water column | A unit of gas pressure |
Logbook
- Safety: rescue drill record
- Unit 1: survey forms; signed designs
- Unit 2: site report; quote; build checklist
- Unit 3: commissioning record; handover sheet
- Unit 4: pipe plan; pressure readings
- Unit 5: diagnosis sheets; service records
- Unit 6: institution sizing; effluent plan; rules poster
- Unit 7: slurry plan; payback; business plan
- Every homework, dated
Self-check quiz
- What do you do if someone collapses in a digester pit?
- At what range of methane in air can an explosion happen?
- How much digester per person does the prison guide use?
- Why build two digesters rather than one very large one?
- Name three common design faults.
- Why must you not fill a new digester to the very top?
- Why fit an H2S scrubber?
- What did after-sales service do in East Africa?
- Is toilet digester effluent safe to put on lettuce?
- How do you stop nitrogen being lost from slurry?
Further reading and sources
Best next reads: your Level 1 manual; the Biogas Digest volume 2; the ICRC prison biogas guide; the Hub's Money Skills manual.
What this module does not cover: entering or repairing inside digesters, large industrial plants, and gas sales licensing.
Every source used in this manual is listed, with links, in the Teacher Copy.